A bearing that is highly rigid and has high rotating precision as well as low heat generation is desired for rotatably supporting the shaft of machine tools in order to improve machining precision. Moreover, in recent years, high-speed stability has been desired in order to improve processing efficiency, so that the equipment can be used at high speeds for long periods off time.
With respect to these characteristics, in order to improve rigidity in the radial direction, use of a cylindrical roller bearing as the bearing has become common. Also, in order to improve the rotating precision, as well as further improve the rigidity in the radial direction, the internal space inside the cylindrical roller bearing is made negative such that a pre-loading is applied to the bearing,
However, applying this kind of pre-load creates extremely severe conditions for a bearing such as a roller bearing making it easy for failure such as wearing or seizure of internal parts to occur.
Therefore, rolling bearings for normal industrial machinery are commonly made such that there is still a positive space inside the bearing during operation, which makes it possible to extend the Raking life of the bearing, as well as suppress the drop in bearing function due to disturbances.
Moreover, it is common for the rolling bearings of machine tools to be operated under lubrication conditions such as adding very small amounts of grease or lubrication oil in order to minimize the heat generated during operation. In other words, it is possible to reduce the agitation resistance of the lubricant and the heat generated due to the agitation resistance by keeping the amount of grease or lubricant to the minimum required amount.
There are many problems that must be solved in order to further increase the rotation speed of a rotating body, such as a shaft that is supported by a roller bearing so that it rotates freely, under the strict operating conditions described above. One such problem is the problem of wear of the copper alloy cage that is standardly used in conventional roller bearings.
That is, when a roller bearing is used under the extremely severe conditions described above, the surface around both the inside and outside peripheral of the cage, or the inner surface of the pockets heavily rub against the peripheral surfaces of the races or against the surfaces of the rollers (rolling surface and end surface).
Therefore, since the copper alloy cage, which is soft compared with the hard steel of the bearing which comprises the races and cage, is worn down, and it becomes easy for abrasion powder to be produced from the cage due to the abrasion. Particularly, if grease is used for lubricating the roller beating, this abrasion powder is mixed with the grease (soils the grease), thus causing a reduction in lubrication by the grease. If lubrication is greatly reduced, there is the possibility that the roller bearing could bun to seizure in a short period of time or that other damage could occur due to wear.
Taking these problems into consideration, in recent years, the use of synthetic resin cages in roller bearings for supporting free rotation of rotating bodies that receive large loads, such as the shaft of machine tools, is becoming more common.
Normally, these synthetic resin cages are formed by injection molding of fiber reinforced resin in which an adequate amount of reinforcement material, such as glass fiber, is intermixed with a synthetic resin, such as a polyamide resin, that has superior friction characteristics (wear resistant). In roller bearings with this kind of synthetic resin cage, it is more difficult for abrasion powder to be generated under the severe conditions described above, and thus it is more difficult for damage such as seizure or severe wear to occur.
However, only by changing the material of the cage in the roller bearing for rotatably supporting a rotating body such as the shaft of the machine tools that receives a large load, specifically by just changing it from a copper alloy to a synthetic resin, it may not be possible to adequately maintain reliability and durability of the rotation support. The reason for this is as follows. For synthetic resins such as a glass-fiber reinforced polyamide resin, the rigidity and breaking strength is less than for a copper alloy, Therefore, if the same shape as a conventional copper alloy cage is used, it is difficult to maintain adequate rigidity and strength. For this reason, the shape and dimensions of the synthetic resin cage are made thicker and larger than the conventional copper alloy cage
Generally, a mold is used in order to form the synthetic resin cage using injection molding, and the shape of the mold is either a radial draw type or axial draw type depending on the shape of the cage to be made. Of these, the axial draw type is made of two mold elements which displaces relative to each other in the axial direction of the synthetic resin cage. Therefore, the shape of the cage to be made is such that the pair of mold elements are removed in the axial direction, or in other words, it must be of a shape such that the pair of mold elements can be separated so that the synthetic resin cage is not scratched after injection molding. On the other hand, the radial draw type of mold comprises a pair of mold elements which move in the radial direction of the synthetic resin cage, and multiple mold elements (usually the same number as there are pockets) that can be freely moved in the radial direction. In this case, it is not necessary that the shape of the synthetic resin cage to be made be such that mold element are removed in the axial direction. In the case of the radial draw type mold, however, manufacture of the mold is complicated, so when compared with the synthetic resin mold made using an axial draw type mold, a high manufacturing cost cannot be avoided.
The synthetic resin cages assembled in the roller bearing have a first and second circular rig sections that are arranged such that they are concentric and parallel with each other and have a space between them. Moreover, multiple columns are aged between the first and second circular ring sections and equally spaced around the circumference, and one end of the columns is formed continuous with the inner side surface of the first ring section, and the other end of the columns is formed continuous with the inner side surface of the second nag section. Also, rollers are rotatably held inside pockets that are formed in the sections surrounded by the opposed surfaces in the circumference direction of the adjacent column sections and the inner side surfaces of the first and second ring sections.
When making a synthetic resin cage for this kind of roller bearing using an axial draw type mold, it is necessary to form. The inner diameter of the 1st ring section so that it is larger than the outer diameter of the second ring section to meet the aforementioned molding restrictions.
On the other hand, if the synthetic resin cage is made using a radial draw type mold, both the first and second ring sections can be made with the same dimensions and shape, It is conventionally supported that if the dimensions and shape of the first and second ring sections are made the same in this way, and if the synthetic resin cage is synthetric with reference to the center along the axial direction, then it. is possible to maintain dynamic balance of the synthetic resin cage when the roller bearing is operated at high speed and the synthetic resin cage is rotated at high speed, and thus to maintain its durability. Therefore, the radial draw type mold has conventionally been used for making the cage built in the roller bearing for rotatably supporting a rotating body that received large loads, such as in the shaft of machine tools, and the synthetic resin cage I having a symmetric shape with reference to the center along the axial direction, as shown in FIG. 31, has been used.
When the synthetic resin cage 1 having a symmetric shape around the center along the axial direction as described above is built into a cylindrical roller bearing which forms the rotation support for a rotating body which receives large loads, such as the shaft of machine tools, the inventors have found through research that not necessarily is it possible to adequately maintain die reliability and durability of the synthetic resin cage 1. The reason for this is as follows.
That is, there is no problem if the roller bearing with synthetic resin cage 1 is installed properly in the rotation support section, however it is not always possible to install it properly. For example, if the adjustment of assembly clearance is inadequate such as when the housing and outlet ring, or the shaft and inner ring are fastened too tightly, there is a possibility that the internal clearance in the roller bearing will shift to the negative side. Moreover, even if the internal clearance of the bearing is proper immediately after assembly, if roller bearing generates excessive heat due to agitation resistance of the lubrication grease during test operation, there is a possibility that the internal clearance of the roller bearing could greatly shift to the negative side while operating
If the internal clearance of the roller bearing greatly shifts to the negative side, for example. if the center axis of the inner ring and the center axis of the outer ring are at an angle with each other due to poor installation tolerance, or poor processing precision of the shaft and housing, there is a possibility that the synthetic resin cage 1 could be damaged. That is, in this case, the rotation of the rollers which make up the roller bearing becomes irregular, causing difference in revolution speed of the rollers in one row to occur. As a result, the rolling surfaces of the rollers with differing revolution speeds between the other rollers are pressed against the opposing columns 4, and abnormal forces act in the circumferential direction at these columns 4.
As mentioned above, in the case of the synthetic resin cage 1 that is made using a radial draw type mold and that is symmetrically shaped with reference to the center in the axial direction, both ends of the columns 4 are firmly joined and supported by the first circular ring section 2 and second circular ring section 3. Also, as described above, the thickness of the synthetic resin cage is somewhat increased and the elastic deformation is small, so it is not possible to sufficiently escape the force applied to the columns 4. Therefore, the stresses generated in the synthetic resin cage 1, for example at the portions where the ends of the columns 4 come together with the first circular ring section 2 and second circular ring section 3, become excessive, and there is a further possibility that the synthetic resin cage could be damaged.